1. Field of The Invention
The invention relates to molecular beam epitaxial (MBE) processing and more particularly to a two-zone electrical furnace for MBE processing.
2. Prior Art
Since 1981 interest in the MBE technique for the growth of mercury cadmium telluride (MCT) has rapidly increased. MCT is an important infrared detector material, potentially suitable for making high resolution focal plane arrays. The interest in the MBE technique for MCT growth stems from the potential for greatly improving the quality of the MCT material compared to that achieved using liquid phase epitaxy or bulk-grown MCT. An improved MCT material would lead to the ability to produce superior infrared focal plane arrays for second-generation imaging systems. The technique should also produce complex device structures such as super lattices or infrared lasers.
Although MBE has been established as a preferred process for the growth of III-V materials such as GaAs and AlAs, the MBE growth of II-VI materials is in its infancy and is significantly complicated by a number of factors. These factors are related to the general instability of MCT and the relatively high vapor pressures of the constituent materials.
One important requirement for the growth of MCT by the MBE process is a stable molecular beam of the constituent materials, Te, CdTe, or Cd. The general approach for producing these molecular beams is to load the pure, clean elemental or compound material into an effusive crucible for insertion into a single zone furnace, typically made from the refractory metal tantalum. A thermocouple, in intimate contact with the bottom of the crucible, serves as the sensor for a temperature controller. The heating elements are set inside a radiator of tantalum foil and are shielded externally by several wraps of tantalum foil. Typically, the furnace is positioned in an MBE system such that it is thermally isolated from other furnaces --which may be operating simultaneously--by a cryoshroud maintained at 77K. Evaporation of the source material at a rate determined by the furnace temperature, creates a molecular beam of the material. The beam is directed toward the growth surface on a substrate supported in the reaction chamber to form an epitaxial layer. The single zone furnace of this design works well for the evaporation of III-V materials such as GaAs or AlAs but not as well for II-VI materials.
The major problem in using a furnace of a conventional single zone design for II-VI materials such as Te, which require an effusion type crucible, is the inherent temperature gradient along the length of the furnace. The region at or near the top of the crucible and adjacent the reaction chamber is cooler than the region at the base of the crucible due to radiative heat losses to the adjacent cryoshroud and to the reaction chamber. The lower temperature promotes recondensation of the II-VI materials at the top of the crucible. In an open crucible, the end result is a gradual clogging at the top of the crucible which leads to a time dependent decrease in the molecular flux being deposited on the substrate. In an effusion type crucible, required for MCT, clogging occurs rapidly, making it virtually impossible to grow compositionally uniform MCT using a conventional single zone furnace.
The problem of furnace clogging is well known in the II-VI community. The few attempts to solve this problem center on providing additional heat to the open end of the furnace. For example, two different types of solutions are known commercially: (1) increasing the resistance of the heaters near the top of the furnace by etch-thinning that part of the heater assembly and (2) installing the heaters only at the top of the furnace. The problem with the first solution is that it only works for a narrow temperature range. Varying the molecular beam flux out of this small temperature window again produces clogging. The problem with the second solution is that a temperature gradient exists in reverse and does not provide a stable beam flux. An additional problem that arises from these solutions is that telluriium reacts with hot tantalum, effectively destroying the furnace in the region of the orifice over time.
A proposed solution has been to create a heated region at the top of the furnace which is independently controlled. This approach has been used in a variety of other applications, such as arsenic crackers. While this approach would appear to be most promising for growing II-VI materials using effusion type crucibles, the known designs are not wholly satisfactory. While achieving a measure of flux stability, and avoiding clogging, precise flux control has been difficult, and the known designs have suffered from limited lifetimes due to their reactivity at high temperatures with tellurium. This reactivity has also tended to contribute impurities to the epitaxial layers formed on the substrate.